In situ Fourier transform IR spectroscopy has been found to be an appropriate tool for the evaluation, monitoring and optimization of the pH dependent and two-step oximation of acetone in water. When carrying out the reaction at 5 degrees C, the intermediate formation of the acetone-hydroxylamine adduct (2) was observed at pH 8.5, while at pH 10, the oxime (3) was formed almost instantly. Identification of the intermediate (2) was promoted by the IR spectrum obtained by B3LYP/6-31++G(d,p)//PCM calculations. At pH similar to 2.5, the protonated form of the oxime (3') developed gradually, but the intermediate (2) could not be observed. The pH dependence of the stability of the intermediate (2) and the (3' ->) 3) and (3 ->) 3') interconversions were studied in separate experiments. An equilibrium was observed in the latter case.